1. Field of the Invention
The present concerns a magnetic resonance imaging scanner, containing a magnetic field system and a radio frequency system as well as a control device, and a method for operating such a scanner, wherein the control device generates a control signal sequence based on a control protocol in order to execute a number of scans and a number of adjustments for the purpose of adjusting the magnetic field system and/or radio frequency systems, wherein at least the scans are predetermined through the control protocol.
2. Description of the Prior Art
Magnetic resonance imaging has become a widely used method for capturing internal images of the body of an examination subject. With this imaging modality, the body to be examined is exposed to a relatively high static magnetic field, e.g. of 1.5 Tesla, which with newer, so-called high magnetic field apparatuses can be high as 3 Tesla. It is then exposed to a radio frequency excitation signal using an antenna device designed for this purpose, which causes nuclear spins of certain atoms to be excited by magnetic resonance when this radio frequency field is altered at a certain flip angle with respect to the magnetic field lines of the static magnetic field. The radiated radio frequency signal resulting from the relaxation of the nuclear spins, the magnetic resonance signal, is then detected with an antenna device designed for this purpose, which may be the same as the transmitting antenna device. The raw data acquired in this manner are then used for the reconstruction of the desired image data. A spatial encoding is superimposed on the static magnetic field during the transmitting and the reading out or detecting of the radio frequency signals by defined magnetic field gradients.
Accordingly, such magnetic resonance imaging scanners contain multiple subsystems that must be activated in a scanning procedure within a predetermined scanning sequence with regard to fixed temporal restrictions. This includes, among others, the above mentioned magnetic field system, which contains a static magnetic field system for generating the static magnetic field, as well as a gradient system for generating the magnetic field gradients, as well a radio frequency system, which contains not only the transmitting and/or receiving antennas, but also the radio frequency apparatuses and receiving channels used for operating the antennas and for processing the received signals.
A magnetic resonance imaging examination typically is composed of numerous independent separate magnetic resonance scanning procedures, e.g. for recording data at various positions within the machine of the patient, in order to acquire magnetic resonance data for different physiological processes or to excite different nuclei, etc. For each separate MRI scanning procedure, the subsystems must be prepared appropriately, and this is done through the adjustment readings. For example, the appropriate resonant frequency and the transmitter reference amplitude suited to an individual scan may be determined. Furthermore, there are numerous additional adjustments for other purposes such as for magnetic field homogenization, for water signal suppression, for setting sensitivity conditions etc. The adjustments are normally carried out by the magnetic resonance system or its control device independently prior to the diagnostic scan desired by the operator in each case. For this reason, the determination of adjustments is normally very specific with regard to the parameters of the actual scanning procedure. Furthermore, the adjustments are normally not only dependent on the magnetic resonance imaging scanner itself, but also on the specific object being examined because the physical conditions that exist within the magnetic resonance imaging scanner, specifically the homogeneity of the magnetic field and the radio frequency field, are significantly influenced by the object being examined.
Normally, the control device is given a specific sequence of scans by an operator using a control protocol that is created by the operator or that is selected from a number of predetermined control protocols that, when necessary, may be altered by the operator. For this purpose, certain adjustments (choices) may be included in the control protocol. Other necessary adjustments are in turn automatically added by the control device as well. A control signal sequence is then generated on the basis of the control protocol, which may then be executed in coordination with the various subsystems of the magnetic resonance imaging scanner, thereby executing the adjustments and the scanning procedures.
A typical example of a scanning sequence is shown in FIG. 2. Three consecutive scanning procedures N1, N2, N3 are executed along a timeline t, each having two adjustments J1.1, J1.2, J2.1, J2.2, J3.1, J3.2. The adjustments J1.1, J1.2, J2.1, J2.2, J3.1, J3.2 in this case are then automatically inserted prior to the respective scans N1, N2, N3 in each case.
A problem occurs when the scanning procedures are to be done consecutively without a pause. One example of this is scanning procedures that need to be executed within a limited period of time, after a contrast agent has been administered to the patient, and the concentration or diffusion of the contrast agent in a certain organ is to be examined using the image data acquired from the magnetic resonance imaging scanner. A similar problem may also occur with objects that move, for example when examining a heart in order to record images in a specific sequence of movement phases. When such scanning procedures should or must be executed directly in sequence, then the adjustments that are automatically executed by the system are disruptive, because the initiation of the subsequent scan intended by the operator is delayed.
A magnetic resonance imaging scanner is described in the DE 198 24 203 C2 with a means for solving this problem, which allows the application of adjustment parameters, which have been executed for another, earlier scanning procedure, to a subsequent scanning procedure, as long as the two scanning procedures are carried out under compatible conditions. Furthermore, it has been suggested in this document that all adjustments within an adjustment sequence be collected, the adjustment parameters be stored, and then all of the adjustments be executed in a subsequent data acquisition sequence.
This process, however, has the disadvantage of being relatively inflexible. Some scanning procedures may not need to be carried out directly in sequence such that an immediately preceding adjustment is possible. Alternatively, the entire scanning may be extended by the unnecessary prioritizing of the adjustments, for example when adjustments must be carried out precisely at the same patient table position as the scans. In this case, the table must pass through the various positions sequentially within the entered adjustment sequence and then again during the actual image data acquisition at a later time.
The result of this is that in practice a recycling of reusable adjustment parameters from previous adjustments is implemented in order to reduce the number of adjustments. A general prioritizing of all of the adjustments then does not occur. This means the current magnetic resonance imaging scanners are still configured such that the control device directs that necessary adjustments take place prior to a scanning procedure when no suitable adjustment parameters are available from previous adjustment readings.
If it is then the case that when a particular scan is to directly follow a previous scan, it is currently the practice is that an empirical determination is made, usually from the scanning protocol of the developer (i.e. the operator or specialist who develops the protocol for the user in advance), as to which of the remaining adjustments for a certain magnetic resonance imaging scanner are to be executed additionally in the processing of a scanning protocol. If it is then determined that, prior to a scanning procedure which is to be carried out directly after another scan procedure, an adjustment shall be executed by the control device, then a suitable previous scanning procedure is modified such than an analogous adjustment is carried out in advance so that the results of the necessary adjustments are available in the memory and thus can be reused, and the system dispenses with the insertion of the undesired adjustments. If a modification of a previous scan is not possible, then a replacement “scanning procedure” is inserted (that in itself is unnecessary), solely to obtain an adjustment reading.
A direct specification of the necessary adjustments for a specific point in time in advance in the scanning protocol by the scanning protocol developer is unfortunately not possible, because the systems are extremely complex and dependant on the machine type and system type, so that as many as 20 adjustments, depending on the scanning procedure, must be carried out in advance. For this purpose, it should be taken into consideration that the precise sequence and in what relationship and which adjustments should be automatically applied by the control device, is not easily identifiable by the operator or the scanning protocol developer due to the complexity of the system. In comparison, it is normally possible for an experienced operator to determine which adjustments need to be carried out in running the program in the framework of a test. For this reason, the developers of the scanning protocol must rely on the previously described empirical protocol development and testing, even when this requires significantly more additional work.
A further problem with a control protocol developed in this manner is that the adjustments are, as noted, system dependant, and this dependency may vary from one model to another by the same manufacturer, or may vary even in the case of system changes within a model. A control protocol developed for one magnetic resonance imaging scanner thus can not simply be used on another magnetic resonance imaging scanner without first checking whether, with the new magnetic resonance imaging scanner, it is the case when two scans that are executed consecutively without a break, no undesired additional adjustments are inserted through the modification of the scanning process. The described empirical method is also particularly critical because changes in the magnetic resonance imaging scanner can induce new adjustments which are not taken into consideration in the previous control protocol. In this case, a scan procedure can be delayed by an unintentionally implemented adjustment—in the worst case, even unnoticed. Such an unintentional or unnoticed adjustment also can lead to faulty results, or at least result in the entire scan needing to be repeated, which is furthermore unpleasant for the patient as well.